1. Field of the Invention
The present invention relates to a positioning system, a positioning method, and a positioning program for estimating a location of a mobile communication terminal.
2. Related Background of the Invention
There are conventionally known methods for using radio waves as radio signals to estimate a location of a mobile communication terminal receiving the radio waves. For example, in cases where the number of radio wave sources is three or more, an available method is to calculate distances from the mobile communication terminal to the sources, to find intersections of circles drawn by setting a center of each circle at the location of each source and using the calculated distance as a radius, and to define the intersections as the location of the mobile communication terminal. Systems of this type include GPS (Global Positioning System), AFLT (Advanced Forward Link Trilateration), and so on.
It is necessary to increase the number of sources depending upon the number of unknowns to be estimated, e.g., deviation of time synchronization, influence from propagation paths of radio waves, and so on. When the number of radio wave sources is less than 3, the foregoing location estimation method is not available. A variety of positioning methods have been proposed as location estimation methods of the mobile communication terminal under such circumstances. One of them is a method of estimating a distance and radio wave direction from a radio wave source to the terminal and estimating the location of the terminal, based thereon. When this method is applied, for example, to a cellular system, sector positioning is used if a base station as a radio wave source is sectorized. In the sector positioning, as shown in FIG. 12 (a), the location of the mobile communication terminal 51 is estimated to be a predetermined position in a sector 50, e.g., the centroid of the sector (location sector) 50 where the terminal is located (which is indicated by point a in the drawing; the same also applies hereinafter). Furthermore, high-accuracy sector positioning is available, as shown in FIG. 12 (b), in a system capable of measuring transmission delays (e.g., RTT: Round Trip Time), intensity attenuations, etc. of radio waves transmitted and received. Since the high-accuracy sector positioning permits us to measure the distance between the base station 52 and the terminal 51, based on the measured RTT or the like, the estimated location is calculated by approximating a signal direction to a location sector direction D being a direction obtained by equally dividing an angle of the sector from the base station 52 (this method is called RTT positioning where the RTT is used).
Furthermore, it is also possible to implement the RTT positioning, based on distances and sector positions from two different base stations 52 based on the RTT, as shown in FIG. 12 (c). This method is to calculate intersections of two circles with the center at the position of each base station 52 and the radius as the distance between each base station and the terminal and to estimate the location of the terminal to be an intersection included in the location sector 50. There is also hybrid positioning in which one of the base stations 52 in the above-mentioned method is replaced by a GPS satellite 53, as shown in FIG. 12 (d). The above-mentioned positioning methods are described, for example, in Yulin Zhao, “Standardization of Mobile Phone Positioning for 3G Systems,” IEEE Communications Magazine, p.p. 108-116, July 2002, and Samir S. Soliman and Charles E. Wheatley, “Geolocation technologies and applications for third generation wireless,” Wireless Communications and Mobile Computing, vol. 2, p.p. 229-251, 2002.